The present invention relates to new aromatic-aliphatic ketones and their use as photoinitiators, in particular for the photopolymerization of ethylenically unsaturated compounds, and as UV-curing agents for printing inks, and to photopolymerizable systems containing such ketones.
Photochemically initiated polymerization reactions have achieved great importance in industry, especially where thin layers are to be cured rapidly. Examples include the curing of coatings of lacquer and resin on paper, metal and plastic and the drying of printing inks. Such photochemical processes are distinguished by a savings in raw material and energy and less pollution of the environment compared with conventional methods for printing and coating articles. Since, generally, none of the reactants in these reactions is capable of absorbing the photochemically active radiation to a sufficient extent, it is necessary to add so-called photoinitiators which are capable of absorbing incident visible or UV radiation and forming active starter radicals when being excited. According to the kind of reaction following photoexcitation two groups are differentiated: One group of initiators after being excited directly decays into active starter radicals which in turn initiate the polymerization. Another group of initiators after being excited first interreacts with a further compound to form the active starter radicals which in turn initiate the polymerization.
Essential criteria for the choice of such initiators are, inter alia, the type of reaction to be carried out, the relationship between the absorption spectrum of the initiator and the spectral energy distribution of the available source of radiation, the solubility of the initiator in the reaction mixture, the stability when stored in the dark of the reaction system to which the initiator has been added, and the effect and acceptability of the residues of the initiator remaining in the end product and/or the products formed therefrom during the photochemical reaction. In particular, the rate of reaction largely depends on the initiator used. Consequently, there has been no lack of attempts to provide new initiators which exhibit an increased reactivity with regard to their ability to initiate either the photopolymerization of ethylenically unsaturated compounds or the curing of photopolymerizable systems.
Benzophenone derivatives, benzoin ethers, benzil ketals, dibenzosuberone derivatives, anthraquinones, xanthones, thioxanthones and .alpha.-halogenoacetophenone derivatives have hitherto chiefly been employed as initiators for the photopolymerization of unsaturated compounds. However, the usefulness of these substances in industry is significantly restricted by a number of disadvantages. These include, inter alia, the low stability when stored in the dark of many photopolymerizable systems to which these initiators have been added, low chemical stability and too low a reactivity with regard to their ability to initiate the photopolymerization of ethylenically unsaturated compounds or the curing of photopolymerizable systems. In addition, many of these known initiators cause yellowing of the polymers prepared using them. This is highly undesirable, especially in the coating of light-colored substrate materials and in the case of printing inks cured by UV radiation. For the latter field of use, the frequently low solubility of the known initiators in the photopolymerizable systems is also a great disadvantage. Since printing inks as a rule contain considerable amounts of colored pigments which absorb a large proportion of the energy irradiated without producing any photochemical activity, a relatively large amount of initiator must be added to these inks. Frequently, the known initiators then partially crystallize out. Apart from the fact that the portions which have crystallized out can no longer have an initiating action, after some time the crystallites formed also damage the printing plates, which consist of relatively soft materials.
Furthermore, dialkoxyacetophenones have been described as photoinitiators in German Offenlegungsschriften Nos. 2,261,383 and 2,730,462, and a number of hydroxyalkylphenones have been described as photoinitiators in German Offenlegungsschrift No. 2,722,264. The disadvantages of the initiators known hitherto are exhibited by these compounds to a considerably lesser extent. Also, these compounds, some of which are liquid, are significantly more soluble in the customary photopolymerizable systems than are most of the initiators known hitherto, which as a rule are solid. Moreover, they have a good stability when stored in the dark and a good chemical stability. In particular, however, when these initiators are used in photopolymerization, yellowing of the polymers is observed to a considerably lesser extent than with the hitherto conventional initiators. Nevertheless, in spite of the increased photoinitiating activity of these dialkoxyacetophenones and hydroxyalkylphenones in comparison with the other known initiators, these compounds still exhibit insufficient reactivity in the initiation of photopolymerizations. Relatively long curing times are still necessary. This prevents optimum utilization of UV-radiation equipment.